Studies on Non-Stoichiometric Strontium Ferrate, SrFeO3-X

Abstract

The oxide perovskites SrMO3,where M is a first-row transition metal,are interesting in that a number of the end members have,unusually,an undistorted simple cubic perovskite structure at room temperature, and also because wide ranges of oxygen non-stoichiometry have been reported for several systems. The most detailed study of the SrFeO3-x system (0 ≤ x ≤ 0.5) reported previously was by MacChesney et al who were the first to prepare the stoichiometric material using high pressures of oxygen, (1). SrFeO3.00 was obtained after equilibration at 550 C and 5,000 psi for one week, or at 335°C and 13,000 psi for 16 hours. Samples containing more than 30% Fe3+ (SrFeO2.84 to SrFeO2.72) were reported to have a simple tetragonal distortion of the cubic unit cell, but no superlattice formation was observed. Below SrFeO 2.72, a two-phase mixture of a perovskite-like phase and a bown-millerite-like phase (SrFeO2.50) was found. Gallagher et al assumed from X-ray powder data that SrFeO2.50 (and BaFeO2.50) were isostructural with CaFeO2.50 (space group Pcmn) (2, 3, 4). MacChesney et al also studied the BaFeO3-x system (5), which is complicated by the presence of mixed hexagonal and cubic stacking of octahedra, although, unlike the oxygen vacancy ordering in SrFeO2.75 such behaviour is readily revealed by X-ray powder diffraction and has been directly imaged in the electron microscope (6). Several different phases in the BaFeO2.5-3.0 region have been observed (7–12) but the only solved structure is the 6H form (cch stacking as in hexagonal barium titanate), determined neutron diffraction (13).